1. Field of the Application
The present application relates the integration of a heat exchanger of a lubrication and/or cooling circuit of an axial turbomachine, more particularly of a turbojet engine, so as to be able to ensure a satisfying cooling power.
2. Description of Related Art
Various mechanical or electrical elements of a turbomachine must be lubricated and/or cooled, such as, for example, certain bearings, certain reducers, and the electrical equipment. To do so, a lubrication circuit is provided. The oil plays the role of a lubricant and also of a heat transfer fluid, which necessitates providing for the heat thus collected to be evacuated. Current turbojet engines generate more and more heat, particularly for the following reasons:                greater and greater weight of the bearing chambers, particularly in engines called “open rotor” equipped with two unducted fans;        the integration of high-power reducers, particularly in the jet prop engines called “Geared Turbofan”, where the fan is made to rotate at a lesser speed than the low-pressure compressor for the purpose of improving performance; and        the integration of new equipment such as, for example, high-power starter/generators.        
For these reasons, the oil-fuel exchangers (FCOC for Fuel cooling Oil cooling) are saturated and require the addition of cooling power via air-oil exchangers (ACOC for Air cooled Oil cooler). Various solutions are available to supply cool air for such exchangers, such as, for example, the use of one or several scoops on the fairing which causes an increase of the drag and an aerodynamic disturbance of the flow of the engine, and, consequently, a decrease of performance.
The dimensioning of these exchangers is carried out for critical operating conditions which generally correspond to running the engine at low-speed whereby a certain level of air cooling capacity is required due to the very low fuel flow rate (other available cool source) and whereby the available air flow rate is particularly low.
On modern bypass engines, a system for bleeding the primary circuit toward the secondary circuit having low-speed, makes it possible to maintain the operating stability of low-pressure and high-pressure compressors by avoiding the surging phenomenon. This discharge system usually comprises a variable opening valve system enabling a controlled bleed from the primary flow to the secondary flow (VBV for Variable Bleed Valve).
The European patent document EP 0146487 A1 discloses the arrangement of an air-oil exchanger (ACOC) arranged in deviation of a discharge passage between the primary flow and the secondary flow of a turbojet engine. The discharge flow rate is controlled by a mechanical system with a sliding valve arranged in a channel connecting the wall of the low-pressure compressor, in the vicinity of its last row of rotor blades, with the wall delineating the internal surface of the secondary flow. The heat exchanger is connected to this channel by way of downstream tapping of the sliding valve so as to receive a portion of the discharge flow rate and, consequently, so as to ensure the cooling of the lubrication circuit oil. A valve is arranged between the discharge flow rate tapping and the heat exchanger so as to be able to control the cooling capacity. This arrangement is interesting insofar as it allows for providing additional cooling of the hydraulic circuit oil. However, its drawback is that cooling is possible only in the presence of a discharge flow rate which is nonexistent at high speed since it would cause a loss of efficiency, unnecessary to the operating of the compressors. Furthermore, this device has drawbacks from the standpoint of head loss due to the air tapping and of the conduit sections. The cooling capacity is, consequently, decreased. Setting the heat exchanger in place is also made difficult because of its location.
The patent document EP 0511770 A1 discloses a device similar than that of the previous document with, however, a main difference: the heat exchanger is arranged directly in the discharge channel, in the vicinity of the wall delineating the internal surface of the secondary flow. A flap in the area of the compressor wall makes it possible to control the discharge flow rate. This device has the advantage of providing a greater cooling capacity than in the teaching of the previous document. However, although the cooling capacity is increased, it remains limited by the discharge flow rate, existing only at low speed (where the engine efficiency is less important).
The patent publication document US 2007/0215326 A1 discloses a retractable air-oil heat exchanger mounted in the wall delineating the internal surface of the secondary flow of a turbojet engine. It is arranged substantially more downstream in the area of the high pressure compressor and of the combustion chamber. This mounting is possible only at this location of the machine because of the space occupied associated to it. Such a solution is not adapted to solve the problem of low speed oil cooling since the secondary flow is too low at low speed to provide satisfying cooling. Although the heat exchanger described in this document allows limiting the head losses of each speed to the minimum necessary to cool the oil, it requires a mechanical actuation system costly in volume and mass.
Although great strides have been made in the area of axial compressors, many shortcomings remain.